docs: add gmeta reference implementation guide

AGENTS.md

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+---
+description: "gmeta reference implementation engineering guide for Claude/Codex agents: Rust architecture, testing, documentation"
+alwaysApply: true
+---
+
+# Gmeta Engineering Guide
+
+This is a reference implementation of the gmeta spec ([docs](https://schacon.github.io/gmeta/))
+
+## Rust Style and Idioms
+
+- Use traits for behaviour boundaries.
+- Derive `Default` when all fields have sensible defaults.
+- Use concrete types (`struct`/`enum`) over `serde_json::Value` wherever shape is known.
+- **Match on types, never strings.** Only convert to strings at serialization/display boundaries.
+- Prefer `From`/`Into`/`TryFrom`/`TryInto` over manual conversions. Ask before adding manual conversion paths.
+- **Forbidden:** `Mutex<()>` / `Arc<Mutex<()>>` — mutex must guard actual state.
+- Use `anyhow::Result` for app errors, `thiserror` for library errors. Propagate with `?`.
+- **Never `.unwrap()`/`.expect()` in production.** Workspace lints deny these. Use `?`, `ok_or_else`, `unwrap_or_default`, `unwrap_or_else(|e| e.into_inner())` for locks.
+- Prefer `Option<T>` over sentinel values.
+- Use `time` crate (workspace dep) for date/time — no manual epoch math or magic constants like `86400`.
+- Prefer guard clauses (early returns) over nested `if` blocks.
+- Prefer iterators/combinators over manual loops. Use `Cow<'_, str>` when allocation is conditional.
+
+## Architecture and Design
+
+- For code that you create, **always** include doc comments for all public functions, structs, enums, and methods and also document function parameters, return values, and errors.
+- Documentation and comments **must** be kept up-to-date with code changes.
+- Do not re-discover Git repositories, instead take them as inputs to functions and methods.
+- Avoid implicitly using the current time like `std::time::SystemTime::now()`, instead pass the current time as argument.
+- Keep public API surfaces small. Use `#[must_use]` where return values matter.
+
+## Testing
+
+- All code must have an appropriate test coverage (proportionate to its complexity and criticality).
+- Do not test external dependencies or trivial getters/setters.
+- Create a mix of unit tests and end-to-end tests.
+- Do not use `anyhow::ensure!` in tests; use panicking assertions (`assert!`, `assert_eq!`, `assert_ne!`) so failures are test panics.
+
+## Committing and Version Control
+
+- If available, always prefer the `but` (GitButler) cli over `git`. Always load the respective skill.
+- Before committing, always run `cargo fmt` and `cargo clippy --fix --allow-dirty` and ensure no warnings remain.

CLAUDE.md

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+AGENTS.md